The invention relates to a method of separating oil, water and other evaporable liquids from drilling mud, bleaching earth, sludge from oil tanks, oil shale, etc.
The problem in connection with recovery of oil from oil-based drilling mud became significant because it is now becoming more and more interesting to utilize such mud as opposed to water based drilling mud. This is due to considerable technical advantages in exploratory drilling as well as in production drilling of oil wells in land based as well as offshore based drilling operations.
Due to the oil content of the mud returned from the drilling hole this mud cannot be freely disposed of in nature, and if this should be done there will be severe requirements as to the treatment of the mud for removing any oil from it.
Especially the fine-grained portion of the drilling mud is problematic. The coarse-grained portion is screened off on oscillating screens and may be washed before being dumped, or residual oil may be burned off. Today, this method is mostly used on installations in the North Sea.
The fine-grained portion from the oscillating screen or the washing process is usually treated in decanters or hydrocyclones, where some oil and water is separated off from the mud.
The residual oil is strongly bonded to the fine-grained portion of the drilling mud by capillary forces, surface tension, and polar bonds, however. For this reason, there is no satisfactory manner of treating the mud.
In experimental distillation of two kinds of mud, one of which mainly consisted of so called cuttings showing a fine division from 1000 .mu.down, and the other consisting almost exclusively of barite with a fine division from 15 .mu.downward, it was found that surprisingly high temperatures were necessary to drive off the oil from the mud. Due to said forces an increase of the boiling point of 100.degree. to 200.degree. C. will occur. When it is desired to drive off the oil from the mud by heating in conventional manners in a distillation plant so high temperatures are, thus, required that part of the oil products are split up and new hydrocarbons are formed.
These conditions are disclosed in Norwegian Patent Application No. 771,423. Reference is also made to U.S. Pat. No. 3,393,951, (hereafter) according to which drilling mud is conveyed on a conveyor belt and is subjected to IR irradiation before the mud is to be dumped into the sea. Because of the temperature conditions, said method has not been used in practice, and oil in the mud cannot be recovered so as to be recycled.
The same is true as regards U. S. Pat. No. 2,266,586, where gas burners are used and the mud is also rinsed with water. It also should be mentioned that water used for preliminary washing the mud must also be purified and, furthermore, results in an increased power consumption for the heating.
U.S. Pat. No. 3,658,015 relates to a combustion oven, where the oil in the mud is only burned off. This method will, indeed, purify the mud sufficiently to make it disposable in the environment, but the oil is not recovered for recirculation in mud preparation. Due to the explosion hazard, this method has not been used in oil installations. It is also observed that it is necessary to supply energy in order to burn off the oil in the mud. Thus, this method does not differ from what is known from conventional drying kilns.
U.S. Pat. No. 3,860,091 relates to a mechanical method of purifying the mud. Said method involves the utilization of separators to remove as much oil as possible, the residual oil being removed by the aid of a detergent. This method is useful but it is very expensive due to the detergent consumption. Also, it does not result in recovering the residual oil after centrifugation due to the above mentioned capillary forces.
The above mentioned Norwegian Patent Application No. 771,423 discloses a method for evaporating oil from the mud that is conveyed through a screw conveyor heated by electrical resistence elements and/or by a heat transfer fluid, which in turn is heated by the aid of electric auxiliary heat exchangers.
This method differs from the method according to the invention primarily by the fact that heat is supplied by the aid of heat exchange mechanisms and that this occurs in such a manner that the above mentioned capillary forces are not destroyed. As mentioned, this results in a requirement of very high temperatures in order to squeeze the oil out of the mud. The temperature is stated to be approximately 260.degree.-360.degree. C., and this fits in with the experiments made during development of the method according to the present invention. It leads to the same problems mentioned as very important, i.e. oxidation or decomposition of the water-gas is not avoided in order to prevent formation of new compounds. According to the above method efforts are made to avoid this disadvantage by stressing the necessity of utilizing a neutral gas in connection with the process. It is also stated that it is a condition of a successful operation that all oxygen or oxidizing gases are carefully avoided during the heating period of the mud in the container. Experiments made by the inventor confirm this.
After evaporation, the waste-gas is conveyed to a condensor condensing the oil gases by direct sprinkling with water. This clearly differs from the method according to the invention, wherein cold oil is used as a condensing medium. Using water produces a considerable problem of separating the water/oil condensate, and it will never be possible to guarantee that the separated water is free of oil.
Since with oil based mud it is desired to use hydrocarbons that are not very toxic (Kero, Somentor 31, TSD 2803, or TSD 2832), there are, in addtion to the explosion hazard, problems with the material of the plant itself and sooting. The latter is most disadvantageous because decomposition products are formed, which cannot be controlled as regards the health hazard. It is an object of the present invention to reduce this problem.
One might assume that if the oil was distilled off from the mud under a vacuum sufficiently low boiling temperatures would be achieved. Experiments showed, however, that not even vacuum distillation results in sufficiently low temperatures.
With the methods of distillation known, inter alia those mentioned above, boiling temperatures of approximately 360.degree. C. are necessary to drive off sufficient oil from the minerals.